1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device such as an IC (Integrated Circuit) device and a LSI (Large Scale Integrated) device, and more particularly, the present invention relates to a SRAM (Static Random Access Memory) device which includes a Poly-Si (polycrystalline silicon) having a very high resistance in the range of about 200 G.OMEGA. to 1000 G.OMEGA., for example.
2. Description of the Related Arts
The inventor of the present invention knows one kind of such a method of manufacturing a semiconductor device, which includes a high-resistance polycrystalline silicon film (referred to as a "high-resistance portion", hereinafter) as shown in FIG. 1.
In this manufacturing method, a high-resistance portion 20 is formed on an Si substrate 22 provided with an SiO.sub.2 (silicon dioxide) film 26 which is formed by means of a CVD (Chemical Vapor Deposition) method, and then, an SiN (silicon nitride) film 21 is deposited to be formed as a protection film on the SiO.sub.2 film 26 and the high-resistance portion 20 by means of the CVD method, so as to accomplish such a structure that the high-resistance portion 20 is covered and thus protected by the SiN film 21. The SiN film 21 protects the high-resistance portion 20 so as not to degrade the high resistance of the high-resistance portion 20 during a subsequent plasma process such as a plasma CVD method to form a CVD film, a RIE (Reactive Ion Etching) process, H.sub.2 (hydrogen) sintering process, and so on.
Then, a BPSG (boron phosphosilicate glass) film 23 is deposited on the overall surface, and is melted in an N.sub.2 (nitrogen) gas atmosphere with a high temperature of 900.degree. C. to 950.degree. C. Then, a contact hole 24 is formed from the BPSG film 23 to the Si substrate 22 by means of a known technique such as an etching technique. Finally, a metal wiring 25 is formed by means of a spattering method. The SiO.sub.2 film 26 having a prescribed uneven surface serves as a foundation pattern on the Si substrate 22.
The above mentioned structure in which the SiN film 21 covers and protects the high-resistance portion 20, has following basic problems.
Firstly, when a process of forming the contact hole 24 is performed by means of the etching technique while the SiN film 21 is preserved, a protruding portion 21a in a peaked shape is formed of the SiN film 21 at the side wall portion of the contact hole 24, as shown in FIG. 1. This is because the etching rate of the SiO.sub.2 film 26 as the basic pattern is different from that of the SiN film 21, during the etching process to form the contact hole 24. Then, in a subsequent process to form the metal wiring 25, the metal is disturbed during entry into the contact hole 24 and maybe separated by the existence of the protruding portion 21a, resulting in a certain possibility that an upper metal portion 25a and a lower metal portion 25b of the metal wiring 25 are electrically disconnected from each other.
Secondary, in case that the SiN film 21 having a thickness about 550 .ANG. (Angstrom), for example, is deposited directly on the high resistance portion 20, when a round etch reaches to the SiN film 21 due to the etching dispersion during this etching process of forming the contact hole 24, the SiN film 21 at this relevant portion is etched during a subsequent contact plasma etching process. This results in an undesirable shoulder as indicated by an arrow A1 in FIG. 2, which shows the status before the metal wiring process of the semiconductor device of FIG. 1, to form a hole 27 where the high-resistance portion 20 is exposed. Thus formed, hole 27 causes a quite serious problem such as an electrical shortage, a degradation of the pressure and voltage resisting characteristic, and so on.
Thirdly, during the depositing and the heat-treating processes of forming the BPSG film 23 on the SiN film 21, the N (nitrogen) content of the SiN film 21 and the B (boron) content of the BPSG film 23 may chemically react with each other to produce a chemical compound such as B.sub.x N.sub.y. nH.sub.2 O etc., which would appear on the BPSG film 23 as an undesirable alien substance to disturb the subsequent processes, causing the degradation of the manufactured semiconductor device.